Performance characteristics of optical fibers can be degraded by environmental elements. For example, the interaction of water with the surface of a silica fiber produces surface modifications which can reduce the strength of the fiber. Also over a period of time, hydrogen can diffuse into an optical fiber and increase the optical loss in a signal carried by that optical fiber.
In order to prevent such interactions, a coating can be applied to the fiber for preventing deleterious environmental elements from interacting with the fiber. Ideally such a coating acts as an impenetrable hermetic barrier between the fiber and the environment. One such coating, e.g., a carbon coating, is applied under stable ambient conditions to the outer surface of a silica cladding of the fiber by inducing decomposition of a suitable carbon containing organic precursor gas, e.g., acetylene, to form a thin carbon film on the fiber surface, as described by F. V. DiMarcello et al., in a U.S. patent application, Ser. No. 098253, filed Sept. 18, 1987, now abandoned. For optimum results, the carbon coating must be applied at a particular thickness within close tolerances. If the coating is too thin, it does not sufficiently limit the penetration of the undesirable environmental elements, such as water and hydrogen. On the other hand, if it is too thick, fiber strength can be reduced by microcracks which can form in the carbon coating when the fiber is under high tensile force.
A need therefore has arisen for a dynamic method to measure and to control the thickness of the coating being applied to the fiber. The method should allow continuous production of the fiber and avoid any direct contact with the fiber. Any interruption of the continuous drawing process is intolerable. Physical contact with an unjacketed optical fiber can damage the surface and reduce the tensile strength of the fiber. In the prior art, however, thickness of a coating has been measured by static off-line metrology, e.g., by electron microscopy or by calculation based on a direct current measurement of resistance in an electrical circuit. These prior art methods require either the interruption of the drawing process, physical contact with the unjacketed optical fiber, or both.